Advances in weapon systems have resulted in soldiers carrying additional gear to enhance combat effectiveness, but at the cost of increased weight. Today, soldiers on combat patrols in Afghanistan typically carry 92 to 105 pounds of mission-essential equipment which includes extra ammunition, chemical protective gear and cold-weather clothing. The overload causes fatigue, heat stress, injury, and performance degradation for soldiers. To ensure that soldiers maintain their readiness, making the load lighter for soldier has become a top priority for the Army.
Despite years of research and development, the Army's weapons and equipment is still too heavy to allow foot soldiers to maneuver safely under fire. One of the heaviest pieces of load for soldiers is the ammunition. Every solider has to carry a lot of ammunition during combat. For example, the weight of 0.50 caliber ammunition is about 60 pounds per box (200 cartridges plus links). It is burdensome for a soldier to move around with heavy ammunition aside from carrying additional gear at the same time. Conventional ammunition cartridges for rifles and machine guns, as well as larger caliber weapons, are usually made from brass, which is heavy, expensive, and potentially hazardous. There exists a need for an affordable, lighter weight replacement for brass ammunition cartridges that can increase mission performance and operational capabilities.
As early as 1960, the U.S. military recognized the benefits of using polymer or polymer composite materials for cartridge case body applications, and since then much research has been carried out by the military and ammunition industry. Previous studies have demonstrated feasibility but have not achieved consistent and reliable ballistic results. Most of the military's and ammunition industry's recent efforts have focused on a two-piece metal (brass) and plastic hybrid cartridge case body design which encountered numerous failures. Testing of a myriad of materials has revealed that the high pressure exhibited by magnum or large caliber rifle ammunition loads at various temperatures gives unacceptable fail rates of the case portion of the cartridge case body of 25% to 75%. Such fail rates are believed due to the high pressure involved during cartridge ignition, such pressures typically being on the order of more than 50,000 psi.
Lightweight polymer cartridge ammunition must meet the reliability and performance standards of existing fielded ammunition and be interchangeable with brass cartridge ammunition in existing weaponry. At the same time, the light-weight polymer cartridge ammunition must be capable of surviving the physical and natural environment to which it will be exposed during the ammunition's intended life cycle. In addition, the polymeric cartridge case bodies should require little to no modification of conventional ammunition manufacturing equipment and methods.
To date, polymeric cartridges have failed to provide satisfactory ammunition with sufficient safety, ballistic and handling characteristics. Most plastic materials, even with a high glass fiber loading, have much lower tensile strength and modulus than brass. Existing polymer/composite cartridge technologies as a result have many shortcomings, such as insufficient ballistic performance, cracks on the case body at its cap, case and/or base, bonding failure of metal-plastic hybrid cases, difficult extraction from the chamber, incompatibility with propellants, insufficient high temperature resistance (burn holes) and chamber constraints produced by thicker case walls.
Other shortcomings include the possibility that portions of the cartridge case body are not flexible or ductile enough for ballistic purposes. Problems associated with the fail rates of many of the ammunition cartridges are believed to be associated with differences between the ductility of cartridge case and the cartridge cap. If not properly manufactured, the cartridge case or cap may explode or otherwise fail upon firing of the ammunition. Weak cartridges having lower modulus pose other problems, such as portions of the cartridge case or cartridge cap breaking off upon firing, or causing the weapon to jam or to be damaged. There is also a danger to the soldier when subsequent rounds are fired or when the casing portions themselves become projectiles.
Prior patents have taught a polyamide resin composition which provides molded articles exhibiting high strength, high modulus, high heat resistance, high toughness, excellent dimensional stability, and high tensile elongation with a small deviation. Examples include nylon-6 polyamide samples derived from ε-caprolactam and montmorillonite which may be injection molded. Other patents have taught injection molded polymeric casing components, wherein the casing may include a bullet end component, a middle body component, and a head end component. The head end component may be made of polyamide and may contain reinforcing materials such as nanoclay. The case component is formed from a material that is more ductile than the material from which the base component, but equal to or less than the ductility of the material from which the cap component is formed. The cap component is said to have an elongation at break at 23° C. (73 F) of greater than 50%.
To overcome the above shortcomings, improvements in cartridge case body design and performance polymer materials are needed. A need further exists for at least a portion of the cartridge to be made of a polymeric nanocomposite material with even greater flexural modulus at a wide range of temperatures.
Nanocomposite technology has become increasing more developed over the recent years. Polymer resins containing well-dispersed layered silicate nanoclays are emerging as a class of nanocomposites that provide significantly enhanced mechanical, thermal, dimensional, and barrier properties. In some nanocomposites, for every 1 wt. % addition of the nanoclays, a property may be increased on the order of 10%.
To date, the most common nanoclay being studied is montmorillonite. In the nanocomposite field, nylon 6 has become the most common polymer used. Generally, a nanocomposite material of layered silicate nanoclays dispersed in a nylon 6 matrix has been produced by either in situ polymerization, in which polymerization takes place after mixing monomer or oligomer with organically modified montmorillonite, or melt compounding, which adds an organically modified montmorillonite into a polymer melt.
While the use of nanocomposite materials of nanoclays dispersed in nylon 6 have improved the existing prior art with respect to certain parts of ammunition cartridges, there are other parts of the ammunition cartridge where using such nanocomposite materials have not be successfully employed. For instance, even with nanocomposites of the type above described, the case portion of the ammunition cartridge still has an unacceptable fail rate. Accordingly, a need still exists for a polymeric nanocomposite material that brings the fail rate of the ammunition to less than 1% in the temperature range from −54° C. to +52° C. (−65 F to +125 F).